1. Technical Field
The present disclosure relates generally to lighting systems. More specifically, the present disclosure relates to a method and system for driving a capacitively coupled fluorescent lamp.
2. Background of the Related Art
Cold cathode fluorescent lamps (CCFL) are widely used to backlight liquid crystal displays (LCD) and for other applications. Different electronic drivers or inverter circuits, for example, current-fed push-pull, voltage-fed push-pull, active clamped Flyback, and voltage-fed half-bridge inverter circuits, have been designed to operate CCFL lamps in high operating frequencies. A typical frequency range is between 20 kHz and 100 kHz. In this way a high frequency voltage is applied in a discharge space within a discharge vessel or tube of the CCFL forming a discharge.
To increase the illuminance of the CCFL, the gas pressure of the rare gas which fills the discharge vessel or tube is increased. After increasing the gas pressure of the rare gas, the current required for discharge is not sufficient if the voltage applied to the CCFL and the high frequency of the voltage are not increased. Therefore, in order to increase the illuminance or lamp power of the CCFL, not only must the gas pressure of the rare gas be increased, but also the voltage and current applied to the CCFL. However, when the applied voltage is increased, there is the danger of discharge creeping on the outer surface of the discharge vessel which can lead to an insulation breakdown of the CCFL.
To overcome the disadvantages of conventional CCFLs, a capacitively coupled fluorescent lamp has been designed where the traditional cathodes (composed of two relatively heavy nickel-plated iron rectangular tabs forming a xe2x80x9cVxe2x80x9d) are replaced by cylindrical ceramic tubes or capacitive coupling structures. Typically, the cylindrical ceramic tubes have an inner diameter of 2.5 mm, an outer diameter of 3.5 mm and a length of 10 mm. Such ceramic tubes with certain dielectric constant and geometry effectively form series capacitance with the positive column of the lamp. The capacitance is not dependent on frequency. With proper material selection and construction, such series capacitance could be designed for the benefit of the electronic driver.
Due to the improvement of the cathodes, the lamp current is increased dramatically, without having to increase the pressure of the filled gas and the voltage applied to the lamp. In fact, when compared to conventional CCFLs, to deliver the same lamp power, the voltage applied to the capacitively coupled fluorescent lamp is less than the voltage applied to conventional CCFLs.
Further, as an effect, the equivalent lamp impedance is greatly reduced. For example, in a preferred design for the capacitively coupled fluorescent lamp, the lamp voltage is 450 V and the lamp current is 20 mA at 50 kHz. Hence, the lamp impedance is approximately 22.5 kOhm compared with approximately 115 Kohm for conventional CCFLs. Therefore, the capacitively coupled fluorescent lamp overcomes the problems associated with the prior art and also offers several advantages over conventional CCFLs.
As indicated above, electronic drivers or inverter circuits are used to operate CCFLs. In the circuit of FIG. 1, the inverter circuit is of voltage-fed-half-bridge type structure with LC resonant tank. The resonant inductor is Lr. The resonant capacitor is formed by the equivalent shield parasitic capacitance and the equivalent output interwinding capacitance of the transformer T1. The CCFL is denoted by its equivalent resistance R1p. The ballast circuit is controlled by an IC.
A typical set of the CCFL structure parameters are: lamp voltage Vlamp≈690 V; lamp current Ilamp≈6 mA; and equivalent shield capacitance Ceq≈7 pF as shown in FIG. 2. Also, a set of circuit design values are: DC input voltage Vin≈12 V; resonant inductor Lr≈6.5 uH; and output transformer turns ratio N=200.
In the circuit shown by FIG. 1, it can be calculated that the lamp equivalent impedance is 115 kOhm in the nominal steady state. At 50 kHz operating frequency, the equivalent impedance of the shield capacitance is 454.7 kOhm. Such very comparable impedance leads to large parasitic capacitance leakage current through the shield with respect to the lamp current. The circuit power losses are then increased. More importantly, it is difficult to design a unified electronic driver for different type of monitors without brutal forced lamp current feedback control.
In most of designed LCD backlight inverters, the inverter circuit output impedance is limited by the circuit internal structure. To achieve lamp stability, the lamp should be properly ballasted. One ballasting scheme for the narrow diameter CCFL is by connecting a small capacitor Cb in series with the lamp as shown in FIG. 3.
The series capacitance as in FIG. 3 is naturally buried in the structure of the capacitively coupled fluorescent lamp. For the above described capacitively coupled fluorescent lamp, by using the series capacitance, the equivalent capacitance Cb is in the range of 1.1-1.4 nF. At an operating frequency of 50 kHz, the corresponding equivalent impedance is in the range of 2.89-2.27 kOhm. It is measured that the dynamic negative impedance of the capacitively coupled fluorescent lamp is in the range of 5.0-7.0 kOhm in steady state with a frequency range of 25-100 kHz.
Due to the high AC voltage across the lamp (xcx9c500 V) and the low input DC voltage (xcx9c12 V), in most LCD monitor applications, high voltage step up output transformer is inevitable. Such high turns ratio usually generates high leakage inductance on the secondary side of the output transformer, i.e., secondary side leakage inductance. When the inverter circuit is connected to the capacitively coupled fluorescent lamp, such leakage inductance naturally forms a series resonant sub-circuit with the ballasting capacitor Cb which usually adversely affects the operation of the capacitively coupled fluorescent lamp.
The lead value of the ballasting capacitor Cb is depending on the lamp type and operating frequencies. In some commercial prototypes, Cb is typically in the range of 15-68 pF. The disadvantage of this implementation is that the output transformer usually needs to be over designed, such that the reactive power could be transferred to the lamp/ballasting capacitor branch.
Accordingly, there is a need for providing a resonant circuit lamp driving scheme which depending on the inverter circuit type, is capable of choosing different operating points for driving the capacitively coupled fluorescent lamp in order to reduce the transformer size, inverter circuit losses and improve the lamp current waveform.
In accordance with the present disclosure, a method and system are provided for driving a capacitively coupled fluorescent lamp which obviates the problems associated with the prior art.
The method and system of the present disclosure provide a resonant circuit lamp driving scheme for driving the capacitively coupled fluorescent lamp which reduces parasitic capacitance leakage current and consequently reduce the inverter circuit losses; compensates the reactive power using the secondary side leakage inductance to have the resonant frequency approximately equal the inverter circuit operating frequency for current source-type driven circuits and consequently reduce the output transformer size, turns ratio and losses; and forms a series resonant sub-circuit with the embedded ballasting capacitor and the secondary side leakage inductance for voltage source-type driven circuits, such that the resonant frequency is substantially less than the inverter circuit operating frequency and the lamp current is properly shaped along with current ballasting, and consequently reduce the turns ratio of the output transformer.
The disclosed system includes a capacitively coupled fluorescent lamp having cylindrical ceramic tubes. The system further includes an electronic driver or inverter circuit for driving the lamp and supply nodes for receiving a supply voltage. The inverter circuit is a conventional inverter circuit, such as, for example, current-fed push-pull, voltage-fed push-pull, active clamped Flyback, and voltage-fed half-bridge inverter circuits, used in conventional CCFLs.
Specifically, the present disclosure provides a capacitively coupled fluorescent lamp package having a capacitively coupled fluorescent lamp; an inverter circuit for driving the lamp; and supply nodes for receiving a supply voltage. A ballast circuit controlled by an integrated circuit may be connected to the inverter circuit for properly ballasting the lamp.
The capacitively coupled fluorescent lamp package includes a resonant circuit lamp driving scheme for driving the capacitively coupled fluorescent lamp. The driving scheme reduces parasitic capacitance leakage current; compensates the reactive power using the secondary side leakage inductance in order to have the resonant frequency approximately equal the inverter circuit operating frequency for current source-type driven circuits; and forms a series resonant sub-circuit with the embedded ballasting capacitor and the secondary side leakage inductance for voltage source-type driven circuits, such that the resonant frequency is substantially less than the inverter circuit operating frequency and the lamp current is properly shaped along with current ballasting.